The present invention relates generally to location detection for mobile terminals using the observed time difference of arrival method, and more particularly, to a reduced complexity approach for obtaining timing estimates needed for position calculations.
In modern mobile communication networks, there is frequently a need to determine the location of a mobile terminal. Many network operators offer location based services to their subscribers to provide information based on the subscriber's current location (e.g., nearest restaurants, gas stations, etc.). Also, federal regulations require mobile operators to determine the location of persons placing emergency calls. Location tracking systems also use wireless devices to track the location of vehicles, such as cars and trucks for fleet operators. Therefore, reliable methods are needed for determining the current position or location of mobile devices.
One location method being proposed for Long Term Evolution (LTE) systems is observed time difference of arrival (OTDOA). The location of a mobile terminal can be determined by measuring the time difference of arrival of a signal transmitted from three or more synchronized cells. To facilitate OTDOA measurements in LTE, a set of reference signals referred to as positioning reference signals (PRS) are transmitted by the cells during a positioning occasion. Positioning occasions typically occur once every 160-1280 subframes (160-1280 ms). Each positioning occasion comprises up to 6 consecutive positioning subframes. The mobile terminal measures the arrival times of the position reference signals from different cells and reports these observed times to the network. Measurements may be performed for up to 18 different cells, at a distance of up to 240 kilometers.
The position reference signals will be received with varying signal-to-interference ratios and with varying time delays. In some instances, assistance information may be provided by the network to narrow the time span in which signals from a particular cell are expected. The accuracy of the assistance information may vary greatly from ±30 meters up to ±120 kilometers. Thus, the search window for detecting the position reference signals is approximately 0.2 microseconds for the best case scenario and approximately 0.8 milliseconds for the worst case scenario. The large expected differences in arrival times, the potentially low accuracy of the assistance information, and the large number of measured cells increases the processing and memory requirements. One way to reduce the number of calculations to be performed is to divide the cells into groups and to perform the calculations one group at a time. Even with this approach, the complexity of the calculations is significant. Therefore, there is interest in new approaches that reduce the complexity of TOA calculations.